Return to Physics IndexRotational Inertia

Corla Jean Wilson-Hawkins Bethune Elementary School

3030 West Arthington

Chicago, Illinois 60612

312-534-6890Objective:

To demonstrate how the resistance of an object to rotation is rotational

inertia.Materials Needed:

This list is for a class size of 32 students divided into groups.

4 ramps 4 meter sticks

4 cans of broth soup covered with paper 8 100 gm weights

4 cans of tomato soup covered with paper masking tape

pendulum balance beam and long pole

hollow wheel solid wheel

turn table for turning (if possible - optional)Strategy:

To begin the lesson, have the students give you several brainstorming

definitions of what they think inertia is. Next discuss what they think

rotational inertia is and write that on the board. After discussing the

vocabulary, have one student from each group sit on the turn table. Spin them

slowly. Using equal weights or books in their hands have them pull their hands

close to their bodies and then extend them away from their body. As they pull

their hands closer to their bodies they will find that they spin faster than

they do when their hands are extended outwards. Use this opportunity to discuss

how the distribution of mass makes a difference in the rotational inertia.

Next, using your groups, take the 4 meter sticks and tape the 100 gram weights

to the bottom of the stick. Put one on each side and tape around it. Have

students collect data on who could balance it vertically the longest with the

weights on the bottom and again with the weights on the top. Have the students

explain why they thought it was easier to balance the weight on the top of the

stick versus the bottom. Some will be good both ways but the norm tends to be

with the weights on the top.

Using the ramps, have your groups take the soup cans of different contents that

are covered with paper and time them as they roll down the ramp. Have them make

a graph charting the time it took and the can that won. Have them to try to

figure out why one can won over the other. After a thorough discussion, uncover

the cans and talk about the contents in the cans and the distribution of the

mass.

Now take the balance beam and have it properly mounted with the wooden stands on

each end. Have a student walk on the beam with their hands in their pockets,

and again with their hands extended holding a long pole or long stick. Talk

about which way was easier to walk. Again reinforce that it is easier to

balance when the rotational inertia is farther away from the axis.

For some additional fun, make a pendulum and push it back and forth on a long

string. Ask the children to tell you how it is moving. Then shorten the string

and let them tell you how it is going. Of course the shorter one moves faster

because it is closer to the axis. At this point the children can make a

pendulum for a hands on activity.

For a final activity take a ring and a solid disk and roll them down a ramp.

They do not have to be the same weight or size because the theory will still

prove itself. Have the children guess which one will come down first. Explain

to them that since the wooden disk is solid, it's mass is closer to the axis

than the hollow ring which has all it's mass on the outer rim.Conclusion:

The conclusion of this lesson would be to have students explain what we did

today in all our demonstrations. Ask them to tell you what they liked best and

what they liked the least. See if any of your students can think of other

examples of rotational inertia that were not used today that can be added to the

lesson.

Evaluation:

Often times when we do a lot of hands on activities, we fail to reinforce the

comprehension version in writing. My evaluation of this process will be a short

quiz that would include all the examples that we had today and to have the

children explain them to me. I would also have them draw some of the demos for

me on paper so that I know who really was aware of what was happening in the

demonstration. I would be looking for 90% accuracy on the quiz.Summary:

EventResults

bottles empty-easy to move full-hard to move

turntable mass at center-easy to rotate mass at edge-harder to do

meter stick wts on bottom hard to balance wts on top easy to balance

balance beam mass at center-easy to fall mass out-easier to stay

pendulums short-swings faster long-swings slower

soup cans tomato solid-rolls fast chicken-liquid rolls slower

disk and ring disk is faster-mass spread ring is slower-mass all at

uniformly throughout the edge (further out)

Reference:

Conceptual Physics- Addison Wesley Paul G. Hewitt 1986 pg 192-204